Exhaust gas purification system

ABSTRACT

An exhaust gas purification system having a diesel particulate filter to purify particulate matter in the exhaust gas G from a diesel engine, wherein a an exhaust gas passage prevention structure or the like is installed in portions where particulate matter in the exhaust gas is collected and accumulated easily, and, the temperature raises easily by the oxidation of accumulated particulate matter, in said diesel particulate filter.  
     The allows to avoid DPF melting damage or breakage, by equalizing the heat distribution of the diesel particulate filter (DPF) and preventing an abnormally high temperature from generating partially, and an exhaust gas purification system of excellent endurance can be provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention concerns an exhaust gas purification systemhaving a diesel particulate filter (DPF: Diesel Particulate Filter:designated as DPF hereinafter) for purifying the exhaust gas bycollecting particulate matter (particulate matter: designated as PMhereinafter) in the exhaust gas of an engine such as diesel engine orthe like.

[0003] The amount of exhaust of the PM exhausted from a diesel engine isregulated more and more strictly together with nitrogen oxides (NOx),carbon monoxide (CO), hydrocarbon (HC) and others year after year, and atechnology for reducing the amount of PM to be exhausted outside, bycollecting the PM by DPF has been developed.

[0004] The filter for directly collecting the PM includes ceramicmonolithic honeycomb shape wall flow type DPF, or fiber shape type DPFwhere ceramics or metals are formed into fiber, or others. The exhaustgas purification system using these DPF are installed in the middle ofthe engine exhaust pipe and purifies the exhaust gas generated in theengine.

[0005] 2. Detailed Description of the Prior Art

[0006] However, the DPF is clogged along the collection of PM, and theexhaust gas pressure (exhaust pressure) rises as the collected amount ofPM increases, thereby requiring to eliminate collected PM from the DPF.Therefore, several methods and systems have been developed.

[0007] Among them, there is a system for eliminating PM by combustionthrough the heating of the DPF by an electric heater or a burner, orback-washing by flowing the air in the opposite direction. However, inthe case of these systems, the fuel efficiency deteriorates, and theregeneration control is difficult, because the PM is burned by supplyingheating energy from outside.

[0008] In addition, in the case of adopting these systems, often twolines of exhaust passage provided with the DPF are installed, and the PMcollection and filter regeneration are repeated alternately, therebytending to increase the size and cost of the system.

[0009] In order to cope with these problems, a continuous regenerationtype DPF system as shown in FIGS. 32 to 34 have been proposed.

[0010]FIG. 32 shows a continuous regeneration type DPF system(NO₂regeneration type DPF system) 1U by intermediate of nitrogen dioxide(NO₂), and the system 1U is composed of an upstream oxidation catalyst3Aa and a downstream wall flow type DPF 10U. The upstream side oxidationcatalyst 3Aa such as platinum or the like oxidizes nitrogen monoxide(NO) in the exhaust gas and NO₂ obtained by the oxidation oxidizes PMcollected by the downstream DPF 10U to obtain carbon dioxide (CO₂,)thereby removing PM.

[0011] As the oxidation of the PM by NO₂ is performed with less energybarrier and at a lower temperature than the oxidation of PM by oxygen(O₂), and thereby with a reduced external energy supply, and moreover,within a short period of time. Therefore, the collection of PM and theoxidation and elimination of PM can be repeated consecutively with asingle DPF utilizing the heat energy in the exhaust gas. In other words,the DPF 10U can be regenerated by removing PM through oxidation, all theway collecting PM continuously by.

[0012] Besides, the continuous regeneration type DPF system (integratedNO₂ regeneration type DPF system) 1V shown in FIG. 33 is an improvementof the system 1U shown in FIG. 32. In the system 1V, the oxidationcatalyst 32A is applied to the wall surface of a wall flow type DPF 10V,and the oxidation of NO in the exhaust gas and the oxidation of PM byNO₂ are performed on the wall surface. In the composition, the system issimplified, by omitting the upstream side oxidation catalyst 3Aa in FIG.32.

[0013] Then, a continuous regeneration type DPF system (DPF systemprovided with the PM oxidation catalyst) 1W shown in FIG. 33 appliesrear metal oxidation catalyst 32A such as platinum (Pt) or the like andthe PM oxidation catalyst 32B to the wall surface of a wall flow typeDPF 10W. The oxidation of the PM is performed on the wall surface. ThePM oxidation catalyst 32B is a catalyst for direct the PM oxidation byactivating O₂ in the exhaust gas, and is composed of cerium dioxide orthe like.

[0014] For the continuous regeneration type DPF system 1W, the PM isoxidized by NO₂ using mainly a reaction of the oxidation catalyst 32A tooxidize NO to NO₂ in a low temperature oxidation range (about 350° C. to450° C.). The PM is oxidized by a reaction of the PM oxidation catalyst32B to oxidize directly the PM by activating O₂ in the exhaust gas, in acentral temperature oxidation range (about 400° C. to 600° C.). Also,the PM is oxidized by O₂ in the exhaust gas in a high temperatureoxidation range (600° C. or more) higher than the temperature ofcombustion of the PM by O₂ in the exhaust gas.

[0015] These continuous regeneration type DPF systems oxidize andeliminate the PM while collecting the PM by a DPF, by lowering the PMoxidation temperature through the use of the PM oxidation by catalyst orby NO₂.

[0016] However, even in these continuous regeneration type DPF systems,it is still necessary to raise the exhaust gas temperature to the orderof 350° C. Consequently, the aforementioned reaction does not generate,and the DPF can not be regenerated by the PM oxidation in an engineoperation state with a low exhaust temperature or in an engine operationstate with low NO exhaust, because the catalyst activity lowers due tothe decrease of the catalyst temperature or NO lacks, and the PMcontinues to be accumulated, causing the problem of DPF clogging.

[0017] For instance, in the idling operation, low speed or extremely lowload operation when the engine break is operated on the downhill, thefuel is burned scarcely, a low temperature exhaust gas flows into theexhaust gas purification system, lowering the catalyst temperature anddeteriorating the catalyst activity.

[0018] Especially, in the case of using a vehicle having a continuousregeneration type DPF system on board is used for door-to-door deliveryservice or the like dominated by the an urban area traveling, the DPF isclogged easily, because the engine is often operated with a low exhaustgas temperature.

[0019] This operating condition is continued, then, the PM isaccumulated without regenerating the DPF. And the clogging of the DPFcauses the raise of the exhaust gas pressure and the deterioration ofthe fuel efficiency. Therefore, in the continuous regeneration type DPFsystem, the engine is controlled to raise the exhaust gas temperature toa temperature activating the catalyst, for eliminating by burning the PMeliminated in the DPF.

[0020] In addition, in the clogging of the DPF, the PM is accumulatedunevenly causing problems. In short, the PM is not accumulated evenlyall over the DPF, but the PM is accumulated more rapidly in an areawhere the gas flow rate is high and the temperature is locally low thanthe other areas in the DPF.

[0021] To be more specific, in an exhaust gas purification system 20X asshown in FIG. 38, a cylindrical wall flow type staggered seal DPF 10X issupported by a DPF holding mat 32 and fixed to a cylinder case 31 forintroducing an exhaust gas G into all of the cells 11 a, 11 b from thetotal upstream surface of the DPF 10X.

[0022] The exhaust gas purification system 20X is devised to introducethe exhaust gas G evenly into all of the cells 11 a, 11 b of the DPF 10Xby diffusing the exhaust gas G, however, the exhaust gas flow rate tendsto increase in the central portion of the DPF 10X and, also, the outerperipheral side of the DPF 10X tends to be cooled down by heatradiation, decreasing the temperature towards the downstream, and thuslowering the temperature by heat radiation. Consequently, the amount ofaccumulation increases in the outer peripheral passage portion, and thedownstream portion of the central portion of the exhaust gas inflowcross-section of the DPF 10X.

[0023] Besides, in the regeneration mode operation, heat generation bythe PM combustion becomes uneven. In short, when the exhaust gas becomeshot during the regeneration operation in the regeneration mode operationor changes of the engine operation state, the collected PM becomes hotand is oxidized by O₂ or NO₂ of the like in the exhaust gas; however, asthe temperature distribution of the exhaust gas flowing in the DPF orthe temperature distribution of the DPF itself being uneven, the timingto start the oxidation of the PM or the heat generated by the oxidationturn to be different according to the place in the DPF.

[0024] In case of a wall flow type DPF 10X as shown in FIG. 35, as itcan be understood from an example of temperature distribution shown inFIG. 37, in the central passage portion, the PM in the exhaust gas iscollected and accumulated easily and, moreover, hot exhaust gas enrichedin NO₂ flows in during the regeneration mode operation. So, in thecentral passage portion, the accumulated PM is oxidized actively, andthe temperature of this portion is raised. Experiments have establishedthat the downstream portion of the central passage portion becomesespecially hot, and the temperature decreases towards the outerperiphery or towards the upstream.

[0025] In the high temperature generation portion, if the DPFtemperature exceeds the heat resistance temperature (about 145° C.) ofthe cordierite or the like forming the DPF, melting damage is generatedlocally and partially. If the melding damage is generated, not only thePM collecting ability lowers, but also the portion susceptible to the PMaccumulation moves around the melting damage portion, and thetemperature of the peripheral portion increased consecutively to expandthe melting damage portion, causing a considerable problem for theexhaust gas purification system.

[0026] In addition, the thermal stress increases if the DPF temperaturedistribution is uneven, and the repeat of this thermal stress generatesthe fatigue destruction of the DPF, decreasing its life, or the catalystendurance temperature being exceeded, the catalyst deterioratesabnormally.

[0027] The following speculation has been made concerning the localtemperature elevation downstream the central portion.

[0028] First, in respect of the PM collection, the exhaust gas flow rateis high in the central portion, the exhaust gas is hot and the PM canburn relatively easily in the exhaust gas while, downstream, the exhaustgas temperature lowers due to heat radiation or others, the PM can becollected easily without burning and accumulated downstream, and theclogging proceeds from the downstream to the upstream.

[0029] In respect of the PM burning, upstream the central portion, theexhaust gas is relatively hot and the PM can burn easily as oxygen forits combustion is supplied abundantly. Consequently, if the PM burnsupstream, the heat generated by its combustion heats the exhaust gasmoving downstream and, at the same time, the heat propagated frommembers of the DPF heats the downstream portion of the DPF, increasingthe temperature of the DPF in its downstream portion.

[0030] The temperature elevation promotes the PM combustion, increasingthe temperature all the more. This is repeated consecutively towards thedownstream, the temperature of DPF and exhaust gas temperature increasestowards the downstream in the central portion, increasing the downstreamtemperature in particular.

[0031] It is considered that, in addition to the partial increase of thePM accumulation amount and partial increase of heat generation, otherfactors such as low heat diffusion due to a low heat conductivity of theDPF material or a low heat capacity of DPF increasing the temperatureeasily, or the like may involved in the partial generation of such ahigh temperature.

[0032] Besides, in a DPF coated with a catalyst used for the continuousregeneration type DPF systems 1V, 1W or others of FIG. 33 or FIG. 34, asthe PM oxidation capacity of the catalyst is activated by the inflow ofthe exhaust gas heated during the regeneration, the PM accumulated inthe DPF upstream area starts the oxidation and combustion. As the PMoxidation and combustion due to the catalyst effect are extremely rapid,a large amount of heat is generated within a short period of time,increasing further the temperature of the exhaust gas during the passagethrough the DPF upstream area.

[0033] As the still heated exhaust gas flows in the downstream area(post-stream area) of the DPF and the downstream portion of the DPF isheated by the heat conducted from the DPF members, the PM accumulated inthe area oxidizes and burns all of suddenly. Consequently, thetemperature of the DPF is raised partially and suddenly in thedownstream area.

[0034] If they try to lower the amount of the PM accumulation in orderto avoid the abnormally high temperature of the DPF, the volume of theDPF increases considerably, making difficult to load it on a vehicle,or, it becomes necessary to perform the regeneration control morefrequently, deteriorating the fuel efficiency extremely.

[0035] On the other hand, it is known that the progress of DPF cloggingcan be delayed, if the DPF temperature can be raised even slightly,because the range of normal engine operation state other than theregeneration mode where the PM in the exhaust gas is burned, can beenlarged. Also, it is known that if the DPF temperature distribution canbe equalized, a local concentration of the PM accumulation can beavoided, allowing to equalize the heat generated during the combustionof collected PM, prevent a partial generation of high temperature andthus melting damage of DPF.

SUMMARY OF THE INVENTION

[0036] The present invention has an object to provide an exhaust gaspurification system of excellent endurance capable of avoiding DPFmelting damage or breakage, by equalizing the heat distribution of thediesel particulate filter (DPF) and preventing an abnormally hightemperature from generating partially.

[0037] In order to achieve the aforementioned object, in the presentinvention, the structure of portions where particulate matter (the PM)accumulates easily and the temperature raises is partially modified,strong and weak catalyst oxidation power is used, the disposition of theexhaust gas communication passage is devised, and so on.

[0038] First, concerning the partial modification of the DPF structure,exhaust gas passage prevention structure, heat absorber, and heatdiffusion portion of high heat conductivity are disposed in the portionsof the DPF where the PM accumulates easily and the temperature raiseseasily.

[0039] Next, concerning the used of strong and weak catalyst oxidationpower, the catalyst oxidation power to be disposed in the DPF iscomposed so as to decrease in order, step-wise or continuously, from theupstream to the downstream of the DPF, or, from the outer peripherytowards the center of the DPF. Besides, the exhaust gas temperature israised by accelerating the oxidation of NO or other components in theexhaust gas by a strong oxidizing catalyst, in the outer peripheralpassage where the exhaust gas cools downs by heat radiation and, at thesame time, the exhaust gas temperature elevation is limited bycontrolling the oxidation of NO or other components in the exhaust gasby a weak oxidizing catalyst, in the central passage where exhaust gastends to become hot.

[0040] Then, concerning devises for the exhaust gas passage, the DPF isheated or kept warm by the exhaust gas, by circulating the total amountof exhaust gas through the outer periphery and the central portion ofthe DPF, and the DPF is kept hot by the heat-retention effect, topromote the PM combustion.

[0041] More in detail, the composition is as follows.

[0042] First, in the exhaust gas purification system, the partialmodifications of portions of the DPF where the PM tends to accumulateeasily and where the temperature tends to raise easily, are performed inthe following compositions.

[0043] 1) In an exhaust gas purification system having a DPF to purifyparticulate matter in the diesel engine exhaust gas, it is so composedof an exhaust gas passage prevention structure installed in a portionwhere the PM in the exhaust gas is collected and accumulated easily and,at the same time, the temperature raises easily by oxidation of theaccumulated PM, in the DPF.

[0044] According to the composition, as the exhaust gas passageprevention structure for preventing the PM in the exhaust gas fromaccumulating is installed in the portion where the temperature raiseseasily when the oxidation of the accumulated PM starts during theregeneration mode operation or the like, the exhaust gas does not flowin the portion. Therefore, the PM does not accumulate in the portion,avoiding the generation of a high temperature, and preventing themelting damage or combustion loss of the DPF.

[0045] 2) Also, in the aforementioned exhaust gas purification system,the DPF is a wall flow type filter having a number of exhaust gaspassages whose periphery is formed of porous wall surface and inlet sideand outlet side of the exhaust gas passage sealed in zigzag, and theexhaust gas passage prevention structure is installed against theexhaust gas passage of the central portion of the exhaust gas inflowcross-section of the filter and, at the same time, the exhaust gaspassage prevention structure is stop sealed both at the upstream sideand the downstream side of the exhaust gas passage.

[0046] The sealing at both sides of the exhaust gas passage of thecentral portion prevents the exhaust gas from passing through thecentral portion of the DPF, the PM from accumulating in the centralportion, and the melting damage due to a partial high temperature fromgenerating.

[0047] And the decrease of the purification rate can be prevented,because the whole exhaust gas is filtered by making it pass through awall having the filtering function.

[0048] 3) Otherwise, in the aforementioned exhaust gas purificationsystem, the DPF is a wall flow type filter having a number of exhaustgas passages whose periphery is formed of porous wall surface and inletside and outlet side of the exhaust gas passage sealed in zigzag, andthe exhaust gas passage prevention structure is installed against theexhaust gas passage of the central portion of the exhaust gas inflowcross-section of the filter and, at the same time, the exhaust gaspassage prevention structure is made solid without the exhaust gaspassage.

[0049] When the structure of the central portion of the DPF is madesolid, the exhaust gas can not pass through the central portion withoutthe exhaust gas passage, preventing the PM from accumulating in thecentral portion, and the melting damage due to a partial hightemperature from generating.

[0050] And, in the case, the heat capacity increases by making thecentral portion solid, allowing the solid portion to absorb and storeheat generated by the combustion of the PM accumulated in thesurrounding portion. Moreover, the solid structure of the centralportion having a heat conductive function, the heat conduction allows todiffuse heat upstream and avoid a local heating of the surroundingportion.

[0051] Besides, as the solid portion is provided with thermal storageand heat conduction functions, heat stored during the DPF regenerationcan be conducted to the upstream side and the surrounding wall surfaces.As the heat conduction functions in a direction to uniform the DPFtemperature in respect of time and space, the combustion of particulatematter in the exhaust gas and collected particulate matter can bepromoted.

[0052] 4) In an exhaust gas purification system having a DPF to purifyparticulate matter in the diesel engine exhaust gas, it is so composedof a heat absorber installed in a portion where the particulate matterin the exhaust gas is collected and accumulated easily and, at the sametime, the temperature raises easily by oxidation of the accumulatedparticulate matter, in the DPF, or in a surrounding portion of theportion.

[0053] The heat absorber is provided with at least one of thermalstorage function and heat conduction function and, preferably, bothfunctions.

[0054] According to the composition, as the heat absorber is installedin the portion where the temperature raises easily when the oxidation ofthe accumulated PM starts during the regeneration mode operation or thelike, or in the surrounding portion of the portion, the generated heatcan be absorbed by the heat absorber. Therefore, the local generation ofa high temperature can be avoided, and the melting damage of the DPF canbe prevented.

[0055] Also, heat stored during the DPF regeneration by the thermalstorage function of the heat absorber can be conducted to the upstreamside and the surrounding wall surface by the heat conduction function ofthe heat absorber, the DPF temperature is uniformed in respect of timeand space, the combustion of particulate matter in the exhaust gas andcollected PM can be promoted even during the normal operation, allowingto reduce the amount of PM accumulating in the DPF.

[0056] Consequently, heat can be absorbed, and, diffused by a thickporous wall surface, even if a quantity of heat is generated momentouslyby a chain combustion of the PM, avoiding a local generation of a hightemperature, and preventing the melting damage of the DPF.

[0057] Moreover, the increase of thickness of the porous wall surfacedecreases the amount of PM accumulating in a portion provided with theporous wall surface, and the heat generated by the combustion decreasesas much.

[0058] Furthermore, since the structure composed of the thick porouswall can be formed easily by changing the type when molding the DPF, theexpensive manufacturing cost can be avoided.

[0059] 5) Also, in the aforementioned exhaust gas purification system,the DPF is a wall flow type filter having a number of exhaust gaspassages whose periphery is formed of porous wall surface and inlet sideand outlet side of the exhaust gas passage sealed in zigzag, and whereinthe heat absorber is formed of a thick porous wall surface in thecentral portion of the exhaust gas inflow cross-section of the filter orthe thick porous wall surface surrounding the central portion.

[0060] The thick porous wall surface can be disposed in ring-like orconcentric form, lattice or double cross form, radical form or the like,and, in combination thereof, in the cross-section namely when viewedfrom the exhaust gas inflow direction.

[0061] According to the composition, a heat absorber can be installed,because the heat capacity increases and the thermal storage quantityalso increases, by forming thick the thick porous wall surface of theexhaust gas passage (cell) in the central portion or of the portionsurrounding the same. Consequently, the temperature rises slowly, evenwhen a quantity of heat is generated momentously by a continuouscombustion of the PM, because the thick porous wall surface can absorbheat. Moreover, the increase of heat passing cross-section area improvesthe heat conductivity and facilitates the heat diffusion. Consequently,a local generation of a high temperature is avoided, allowing to preventthe melting damage of the DPF.

[0062] 6) In an exhaust gas purification system having a DPF to purifyparticulate matter in the diesel engine exhaust gas, it is so composedof a heat diffusing member, presenting a higher heat conductivity thanthe DPF, disposed in contact with a portion where the particulate matterin the exhaust gas is collected and accumulated easily and, at the sametime, the temperature raises easily by oxidation of the accumulatedparticulate matter, in the DPF.

[0063] The DPF for purifying the PM in the exhaust gas is made porouswith a ceramics such as cordierite or the like, presents a relativelylow heat conductivity and an insufficient heat diffusion, and a localgeneration of a high temperature is frequent. Therefore, the dispositionof the heat diffusing member in contact with the hot portion lowers thetemperature of the hot portion through heat diffusion and avoids themelting damage of the DPF.

[0064] At the same time, it is intended to equalize the temperature,especially to equalize the radial temperature distribution bytransferring heat to the low temperature portion of the DPF, through theheat diffusing member. Whereby, the PM accumulation speed or thecombustion speed can be uniformed. Moreover, portions where thetemperature of the DPF is relatively high can be increased in order topromote the combustion of the PM in the exhaust gas or collected PM,allowing to decrease the amount of PM accumulation.

[0065] Consequently, an uneven PM accumulation or an uncontrolledcombustion of the unevenly accumulated PM, then, the melting damage ofthe DPF provoked by the uncontrolled combustion of the PM or the thermaldeterioration of the catalyst carried by the DPF can be avoided.

[0066] In the simplest composition of the heat diffusing member is anaeration member composed of mesh or porous flat plate, and the same isinstalled in contact with the downstream side face including thedownstream central portion (in case when the DPF is a wall flow typefilter) where the temperature raises easily, and it is so composed todiffuse the heat of the central portion towards the peripheral portion,in the downstream.

[0067] 7) Then, in the aforementioned exhaust gas purification system,the heat diffusing member is composed using metal, silicon nitride, andsilicon carbide as material.

[0068] The heat diffusing member is preferably made of a materialpresenting a high heat conductivity and an excellent heat resistance,and aluminum titanate or other metals, silicone carbide, siliconenitride, alumina or others ceramics are particularly preferable.

[0069] 8) In the aforementioned exhaust gas purification system, the DPFis a wall flow type filter having a number of exhaust gas passages whoseperiphery is formed of porous wall surface and upstream side anddownstream side of the exhaust gas passage sealed in zigzag, and whereinthe heat diffusing member is composed of a stop sealing plate forsealing the downstream side in zigzag.

[0070] In case where the DPF is a wall flow type filter, the downstreamstop seal can be formed with a heat diffusing member. The stop sealingplate is composed by sticking with adhesive to the DPF downstream madeof cordierite or the like, and the adhesive absorbs the difference ofheat dilatation of the DPF and the stop sealing plate.

[0071] 9) Also, in the aforementioned exhaust gas purification system,the heat diffusing member is composed of a member having a filterfunction.

[0072] For the upstream of the DPF where it is desired to burn PM byraising the temperature, a material presenting a low heat conductivityand an excellent thermal storage, and also, a low heat capacity and anexcellent heat-up ability, namely cordierite or the like is preferable,in order to raise the temperature relatively rapidly. Then, for thedownstream side where the temperature of the exhaust gas and the DPFincreases due to the PM combustion in the upstream, in contrast, aceramic material such as silicone carbide, alumina or the likepresenting a high heat conductivity and an excellent thermal diffusion,and also, a large heat capacity and a low heat-up ability, ispreferable, in order to avoid an abnormally high temperature.

[0073] The composition allows to raise the temperature relativelyrapidly, in the upstream DPF, for accelerating the PM combustion, and todiffuse and store the heat, in the downstream DPF, for avoiding anabnormally high temperature.

[0074] Besides, the DPF and the heat diffusing member having the filterfunction are bonded in a way to transfer heat regularly; however, as thethermal expansion rate is different in general, they are both bondedwith an adhesive, and the adhesive absorbs the difference in thermalexpansion.

[0075] 10) In the aforementioned exhaust gas purification system, bothof the DPF and the heat diffusing member are composed of a wall flowtype filter having a number of exhaust gas passages whose periphery isformed of porous wall surface, wherein the upstream side and thedownstream side of the exhaust gas passage are sealed in zigzag.

[0076] In case where a first normal DPF presenting a relatively low heatconductivity and a second DPF (heat diffusing member having filterfunction) composed of a heat diffusing member presenting a relativelyhigh heat conductivity are made of a wall flow type filter which is stopsealed in zigzag, the composition becomes simple, and the shape of thebonding face can be made simply as a shape easy for the exhaust gas toflow.

[0077] Next, the use of strong and weak catalyst oxidizing power isperformed in the following compositions.

[0078] 11) The exhaust gas purification system of the present inventionis an exhaust gas purification system having a DPF to purify particulatematter in the exhaust gas from a diesel engine by using a catalyst,wherein the oxidation power of the catalyst to be disposed in the DPF iscomposed so as to decrease in order, step-wise or continuously, from theupstream to the downstream of the DPF.

[0079] 12) In addition, the exhaust gas purification system of thepresent invention is an exhaust gas purification system having a DPF topurify particulate matter in the exhaust gas from a diesel engine byusing a catalyst, wherein the oxidation power of the catalyst to bedisposed in the DPF is composed so as to decrease in order, step-wise orcontinuously, from the outer periphery towards the center side of theDPF.

[0080] 13) Otherwise, the exhaust gas purification system of the presentinvention is an exhaust gas purification system having a DPF to purifyparticulate matter in the exhaust gas from a diesel engine by using acatalyst, wherein the oxidation power of the catalyst to be disposed inthe DPF is composed so as to decrease in order, step-wise orcontinuously, from the outer periphery of the upstream end towards thecentral portion of the downstream end of the DPF.

[0081] 14) Moreover, in the aforementioned exhaust gas purificationsystem, the oxidation power of the catalyst is changed in three or morestages, or, continuously and, at the same time, the portion of thelowest oxidation power of the catalyst is formed by not disposing thecatalyst.

[0082] 15) There, the aforementioned exhaust gas purification system canbe applied to an exhaust gas purification system, wherein the DPF is awall flow type filter having a number of exhaust gas passages whoseperiphery is formed of porous wall surface and, an inlet side and anoutlet side of the exhaust gas passage sealed in zigzag.

[0083] The change of the oxidation power of these catalyst can berealized by changing the kind of catalyst, or even if the same catalystis used, by modifying the catalyst support density during the coating.

[0084] There, heat generated by the PM oxidation and combustion issupplied to a portion where a catalyst of low oxidation power isdisposed, through heat conduction or others from the heated exhaust gas,the upstream side and the outer periphery side to heating up theportion, allowing to activate sufficient even a catalyst of wealoxidation power, oxidize and burn PM.

[0085] Moreover, as the oxidation power of the catalyst in the portionis weak, a rapid PM oxidation and combustion are avoided, preventing thetemperature of the portion from raising rapidly. As a result, DPFmelting damage, combustion loss, destruction or catalyst deteriorationis also prevented.

[0086] The change in the DPF of the oxidation power of the catalyst tobe applied to the wall of cells of these DPFs can prevent a local hightemperature from generating by avoiding an abnormal combustion of PM, inthe case of DPF regeneration consisting in elimination by oxidation ofPM collected and accumulated in the DPF, allowing to prevent DPF meltingdamage, combustion loss, destruction or catalyst deterioration.

[0087] 16) In an exhaust gas purification system comprising an oxidationcatalyst converter in the upstream of the exhaust gas passage and a DPFto purify particulate matter in the exhaust gas from a diesel engine inthe downstream, the oxidation catalyst converter and the DPF are formedinto a cylinder-shape in which the exhaust gas enters in the upstreamend portion and exits from the downstream end portion and, at the sametime, it is so composed that the outer peripheral passage portion of theoxidation catalyst converter supports a strong oxidation catalyst, andthe central passage portion a weak oxidation catalyst respectively.

[0088] For these strong oxidation catalyst and weak oxidation catalyst,the one presenting a stronger oxidation catalyst effect is called strongoxidation catalyst, and the other presenting a weaker oxidation catalysteffect is called weak oxidation catalyst in comparison of two catalysts.There, both catalysts can be formed by using different kinds ofcatalyst, or the same kind of catalyst, by changing the support densityor support concentration.

[0089] According to the composition concerning a continuous regenerationtype DPF system such as NO₂ regeneration type DPF, as the outerperipheral passage portion where the exhaust gas temperature tends tolower easily through heat radiation or the like supports a strongoxidation catalyst, a high oxidation effect promotes the oxidation of PMor NO components or others in the exhaust gas, and raises the exhaustgas temperature largely. On the other hand, the central passage portionwhere the exhaust gas temperature tends to raise easily supports a weakoxidation catalyst, a low oxidation effect retards the oxidation of PMor NO components or others in the exhaust gas, and raises the exhaustgas temperature slowly.

[0090] Consequently, the exhaust gas which has the higher temperatureand is rich in NO₂ flows in the outer peripheral passage, and theexhaust gas which has the lower temperature and is poor in NO₂ flows inthe central side passage, enters with central side passage, when theexhaust gas passed through the oxidation catalyst converter enters theDPF.

[0091] Therefore, during the normal mode operation, an exhaust gashotter in comparison to the central portion flows in the outerperipheral portion which tends to cool down easily through a large heatradiation, in the DPF, equalizing the temperature distribution in theDPF viewed as a whole. The equalization enlarges the area equal orsuperior to a fixed temperature allowing to burn PM in the exhaust gas,in the DPF, and increases the amount of PM burned without beingcollected.

[0092] Also, in the regeneration mode operation for increasing NOcomponent and raising the exhaust gas temperature, the oxidationreaction is promoted in the outer peripheral portion of the oxidationcatalyst converter, raising the temperature of the exhaust gas passingthrough the DPF outer peripheral passage portion, and increases NO₂.Consequently, collected PM burns rapidly and the temperature elevationbecomes earlier.

[0093] On the other hand, as the oxidation reaction is limited in thecentral passage portion, the temperature elevation of the exhaust gaspassing through the DPF central passage is controlled, and NO₂ is alsolimited. Therefore, collected PM burns slowly, preventing an abnormallyhigh temperature from generating.

[0094] Consequently, PM oxidation reaction is also promoted in the outerperipheral side and limited in the central portion for a gentle reactionduring the regeneration mode operation in the DPF and, therefore, thetemperature distribution in a DPF presenting a relatively high heatradiation in the outer peripheral side will be equalized when viewed asa whole.

[0095] There, as the equalization of the DPF temperature distributionenlarges the area equal or superior to a fixed temperature allowing toburn PM in the exhaust gas in the oxidation catalyst converter or theDPF, during the normal engine operation, and enlarges the area where thePM in the exhaust gas is burned without being collected, and, in theregeneration mode operation, an abnormally high temperature will not begenerated, and the melting damage can be avoided.

[0096] Moreover, the equalization of temperature distribution in theoxidation catalyst converter and the DPF, the magnitude of generatedheat stress is also limited, in a way to improve the enduranceperformance of the exhaust gas purification system, and its life becomeslonger.

[0097] 17) Or, in an exhaust gas purification system having a DPF topurify particulate matter in the exhaust gas from a diesel engine in theexhaust gas passage and disposing an oxidation catalyst in the exhaustgas passage of the DPF, wherein the DPF is formed into a cylinder-shapein which the exhaust gas enters in the upstream end portion and exitsfrom the downstream end portion and, at the same time, it is so composedthat the outer peripheral passage portion of the DPF supports a strongoxidation catalyst, and the central passage portion a weak oxidationcatalyst respectively.

[0098] In the composition concerning a continuous regeneration type DPFsystem such as integrated NO₂ regeneration DPF system, DPF system withPM oxidation catalyst or others also, the oxidation of PM and othercomponents such as NO in the exhaust gas or the oxidation of collectedPM is promoted in the outer peripheral passage portion supporting astrong oxidation catalyst and limited in the central passage portionsupporting a weak oxidation catalyst, in the DPF, the temperaturedistribution in the DPF will be equalized when viewed as a whole.

[0099] Consequently, the area allowing to burn PM in the exhaust gasduring the normal engine operation, and the melting damage due to anabnormally high temperature combustion which is often generated inregeneration mode operation can also be avoided.

[0100] 18) Then, in the aforementioned exhaust gas purification system,the strong oxidation catalyst is made of a rear metal system oxidationcatalyst, while the weak oxidation catalyst is made of an oxide systemoxidation catalyst. According to the composition, the strong oxidationcatalyst and the weak oxidation can be supported respectively in theexhaust gas passage by the oxidation catalyst converter, filter withcatalyst, or the like.

[0101] Besides, even when the same oxidation catalyst is used, bychanging the concentration or density supported by the same, the highersupported concentration can be taken as strong oxidation catalyst, whilethe lower supported concentration can be taken as weak oxidationcatalyst.

[0102] 19) In the aforementioned exhaust gas purification system, thetransversal cross-section area of the outer peripheral passage portionsupporting the strong oxidation catalyst is made into 0.5 to 1.0 time ofthe transversal cross-section area of the central passage portionsupporting the weak oxidation catalyst.

[0103] If the portion designed to support a strong oxidation catalystmade of a rear metal system oxidation catalyst and the portion designedto support a weak oxidation catalyst made of an oxide system oxidationcatalyst are set to the ratio, the oxidation efficiency by the catalysteffect and a satisfactory temperature distribution formation in the DPFcan be maintained in a well-balanced state.

[0104] Namely, when the area ratio is set less than 0.5 times, thecentral passage portion of the DPF becomes excessively hot, the portionof weak oxidation catalyst becomes dominant decreasing the oxidationefficiency, and enlarging the portion supporting the oxidation catalyst.While, if the area ratio exceeds 1.0 time, outer peripheral passageportion of the DPF becomes excessively hot, causing problems ofheat-retention or others.

[0105] 20) And, it can be applied more effectively to a case where theDPF is a wall flow type filter having a number of exhaust gas passageswhose periphery is formed of porous wall surface and an inlet side andan outlet side of the exhaust gas passage sealed in zigzag.

[0106] Other than the wall flow type filter, the DPF formed into acylinder-shape in which the exhaust gas enters in the upstream endportion and exits from the downstream end portion includes also metalfilter or SiC filter.

[0107] It should be noted that oxidation catalyst may be supported bythe oxidation catalyst converter or the DPF, by clearly dividing thesupport range of strong oxidation catalyst and the support range of weakoxidation catalyst, however, it can also be supported in a way totransit progressively form the strong oxidation catalyst to the weakoxidation catalyst step-wise or continuously from the outer peripheryside to the central side.

[0108] Then, inventions for the exhaust gas passage are performed by thefollowing composition.

[0109] 21) In an exhaust gas purification system, wherein the DPF topurify particulate matter in the exhaust gas from an engine is composedof a wall flow type filter having an inlet side and an outlet side of aplurality of cells whose periphery is formed of porous wall surfacesealed in zigzag, and wherein an outer peripheral passage is disposed inthe outer peripheral portion of the DPF and a central passage in thecentral portion respectively, and it is so composed that exhaust gaspasses through the gas inlet passage, the outer peripheral passage, thecells, the central passage and the gas exit passage in this order.

[0110] The DPF may be a normal DPF, or a DPF with a catalyst such asoxidation catalyst, namely catalytic DPF.

[0111] Also, the outer peripheral passage can be formed simply byadopting a composition wherein stop seals before and after the cell inan area along the outer periphery of the cylindrical body are removed,however a composition to provide a passage between the outer peripheryof the DPF and the case of the exhaust gas purification system, namely,to dispose a passage outside the DPF may also be adopted.

[0112] If the outer peripheral passage is composed to be installed outof the DPF, portions important for the purifying and more vulnerablethan the other portions can be protected and even when the outside ofthe exhaust gas purification system is damaged during the transport orinstallation of the exhaust gas purification system, the purifyingcapacity will not be damaged.

[0113] The central passage also can be formed simply by adopting acomposition wherein stop seals before and after the cell in an areaalong the central axis of the DPF are removed, however, the DPF may beformed hollow, for using the hollow portion as central passage.

[0114] According to the composition, the whole quantity of exhaust gasflows in the outer peripheral side of the DPF and then passes throughthe cell of the DPF and further passes through the central passageallowing to heat or keep hot the DPF from the peripheral side with theexhaust gas passing through the outer peripheral passage of the DPF andtherefore to uniform the temperature of the whole DPF.

[0115] The heat-retention effect allows to keep high the temperature ofthe DPF and the temperature of the exhaust gas passing through the cellin the DPF and, thereby, to promote the combustion of PM in the exhaustgas or collected PM. Therefore, the amount of PM accumulation during thenormal operation can be reduced. In addition, the period of time ofregeneration mode operation for oxidizing and removing collected PM canalso be reduced. Then, as the whole quantity of exhaust gas passesthrough the cell of the DPF, the purifying efficiency is prevented fromlowering.

[0116] Moreover, in an exhaust gas purification system, as a centralpassage is installed in the central portion of the DPF which tends tobecome hot to let the exhaust gas pass through without collecting PM,the melting damage of the DPF in the portion can be avoided.

[0117] 22) In the aforementioned exhaust gas purification system, thedisposition of oxidation catalyst in the outer peripheral passagepermits to heat the outer peripheral side of the DPF with oxidationreaction heat due to the oxidation catalyst, and, to economize the spacefor disposing the oxidation catalyst.

[0118] In the case, in the case of outer peripheral passage formed byremoving stop seals before and after the cell in an area along the outerperiphery of the DPF, the oxidation catalyst is supported by the wallsurface of the cell; however, in the case of installing a passageoutside the DPF, it is preferable to compose as follows.

[0119] 23) In the aforementioned exhaust gas purification system, fordisposing an oxidation catalyst in the outer peripheral passage, theoxidation catalyst is supported by a ceramic or metal honeycombstructure formed separately from the DPF, and the honeycomb structure isdisposed in the outer peripheral passage.

[0120] In the case, as the oxidation catalyst support step can beseparated form the DPF formation step, the oxidation catalyst can bedisposed in the outer peripheral passage relatively easily.

[0121] 24) In the aforementioned exhaust gas purification system, arelief valve is disposed at least one of a passage wall between anoutside chamber communicating the outer peripheral passage with the celland the gas exit passage, or a passage wall between an inside chambercommunicating the cell and the central passage, and the gas inletpassage, so that the exhaust gas bypasses the filter, when the cloggingof the filter progresses and increases the pressure.

[0122] The disposition of the relief valve, allows to avoid an excessiveelevation of the engine exhaust pressure, when the filer is cloggedexcessively due to an abnormal accumulation of particulate matter. Inaddition, if the position of the relief valve, is positioned at theposition of the aforementioned passage wall, a canalization fordisposing the relief valve becomes unnecessary, allowing to make theexhaust gas purification system more compact.

[0123] 25) Otherwise, in an exhaust gas purification system wherein theDPF to purify particulate matter in the exhaust gas from an engine iscomposed of a wall flow type filter having an inlet side and an outletside of a plurality of cells whose periphery is formed of porous wallsurface sealed in zigzag, and wherein an outer peripheral passage isdisposed in the outer peripheral portion of the DPF and a centralpassage in the central portion respectively, and it is so composed thatexhaust gas passes through the gas inlet passage, the central passage,the cells, the outer peripheral passage and the gas exit passage in thisorder.

[0124] 26) In the exhaust gas purification system, an oxidation catalystis disposed in the central passage.

[0125] 27) Then, for disposing an oxidation catalyst in the centralpassage, the oxidation catalyst is supported by a ceramic or metalhoneycomb structure formed separately from the filter, and the honeycombstructure is disposed in the central passage.

[0126] 28) Further, in the aforementioned exhaust gas purificationsystem, a relief valve is disposed at least one of a passage wallbetween an outside chamber communicating the cell and the outerperipheral passage and the gas inlet passage, or a passage wall betweenan inside chamber communicating the central passage and the cell, andthe gas exit passage, so that the exhaust gas bypasses the filter, whenthe clogging of the filter progresses and increases the pressure.

[0127] The outer peripheral passage or central passage in thesecompositions can be formed similarly as the above-mentioned exhaust gaspurification system, and the composition is different from theabove-mentioned exhaust gas purification system in that the exhaust gaspasses first through the central passage, then passes in the cell andpasses through the outer peripheral passage, and, that the portion wherethe oxidation catalyst is disposed is the central passage.

[0128] In these compositions, as the central passage is upstream, and,the oxidation catalyst is also disposed in the central passage, thecentral portion side is heated by the heat of the exhaust gas and theheat generated by the oxidation reaction of substances in the exhaustgas and, thereafter, the exhaust gas having passed through the cellflows through the outer peripheral passage and the peripheral portion ofthe DPF can be kept hot and heated with the exhaust gas.

[0129] 29) A disposition of a thermal insulation layer on the outersurface of the aforementioned exhaust gas purification system canenhance the thermal retention effect and, at the same time, preventthermal damage of peripheral vehicle components disposed outside theexhaust gas purification system by the heat from the exhaust gas.Besides, the thermal insulation layer having also a normal noisereduction effect can serve for prevention of noise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0130]FIG. 1 is a schematic composition drawing of a DPF of an exhaustgas purification system of a first embodiment according to the presentinvention, (a) is a perspective view including a partial cross-section,(b) is a front view, and (c) is a back view;

[0131]FIG. 2 is a schematic lateral cross-section of the DPF of FIG. 1;

[0132]FIG. 3 is a schematic composition drawing of a DPF of an exhaustgas purification system of a second embodiment according to the presentinvention, (a) is a perspective view including a partial cross-section,(b) is a front view, and (c) is a back view;

[0133]FIG. 4 is a schematic lateral cross-section of the DPF of FIG. 3;

[0134]FIG. 5 is a schematic composition drawing of a DPF of an exhaustgas purification system of a third embodiment according to the presentinvention, (a) is a front view, (b) is a back view and (c) is a lateralcross-section;

[0135]FIG. 6 is a schematic composition drawing of a DPF of an exhaustgas purification system of a fourth embodiment according to the presentinvention, (a) is a front view, and (b) is a back view;

[0136]FIG. 7 is a schematic composition drawing of a DPF of an exhaustgas purification system of a fifth embodiment according to the presentinvention, (a) is a front view, and (b) is a back view;

[0137]FIG. 8 is a schematic perspective view of a DPF of an exhaust gaspurification system of a sixth embodiment according to the presentinvention including a partial cross-section;

[0138]FIG. 9 is a schematic lateral cross-section of the DPF of FIG. 8;

[0139]FIG. 10 is a schematic perspective view of a DPF of an exhaust gaspurification system of a seventh embodiment according to the presentinvention including a partial cross-section;

[0140]FIG. 11 is a schematic lateral cross-section of the DPF of FIG.10;

[0141]FIG. 12 is a schematic perspective view of a DPF of an exhaust gaspurification system of an eighth embodiment according to the presentinvention including a partial cross-section;

[0142]FIG. 13 is a schematic lateral cross-section of the DPF of FIG.12;

[0143]FIG. 14 is a schematic lateral cross-section showing anotherexample of the DPF of the exhaust gas purification system of the eighthembodiment according to the present invention;

[0144]FIG. 15 is a schematic lateral cross-section showing anotherexample of the DPF of the exhaust gas purification system of the eighthembodiment according to the present invention;

[0145]FIG. 16 is a schematic lateral cross-section showing anotherexample of the DPF of the exhaust gas purification system of the eighthembodiment according to the present invention;

[0146]FIG. 17 is a schematic isothermal diagram showing a state oftemperature distribution in a lateral cross-section of the DPF of theexhaust gas purification system of the sixth embodiment according to thepresent invention;

[0147]FIG. 18 is a schematic perspective view of a DPF of an exhaust gaspurification system of a ninth embodiment according to the presentinvention including a partial cross-section;

[0148]FIG. 19 is a schematic perspective view of a DPF of an exhaust gaspurification system of a tenth embodiment according to the presentinvention including a partial cross-section;

[0149]FIG. 20 is a schematic perspective view of a DPF of an exhaust gaspurification system of an eleventh embodiment according to the presentinvention including a partial cross-section;

[0150]FIG. 21 is a flow chart diagram showing an example of regenerationcontrol flow of a continuous regeneration type DPF system according tothe present invention;

[0151]FIG. 22 is a lateral cross-section showing a schematic compositionof an exhaust gas purification system of a twelfth embodiment accordingto the present invention;

[0152]FIG. 23 is a schematic perspective view of an oxidation catalystconverter of FIG. 22 including a partial cross-section;

[0153]FIG. 24 is a lateral cross-section showing a schematic compositionof an exhaust gas purification system of a thirteenth embodimentaccording to the present invention;

[0154]FIG. 25 is a lateral cross-section showing a schematic compositionof an exhaust gas purification system of a fourteenth embodimentaccording to the present invention;

[0155]FIG. 26 is a schematic composition drawing of an exhaust gaspurification system of a fifteenth embodiment according to the presentinvention, wherein an outer peripheral passage is disposed outside theDPF;

[0156]FIG. 27 is a schematic composition drawing of the exhaust gaspurification system of the fifteenth embodiment according to the presentinvention, wherein the outer peripheral passage is formed by removingstop seals from the outer peripheral portion of the DPF;

[0157]FIG. 28 is a schematic composition drawing of the exhaust gaspurification system of the fifteenth embodiment according to the presentinvention, wherein a honeycomb structure is inserted in the outerperipheral passage provided separately from the DPF;

[0158]FIG. 29 is a schematic composition drawing of an exhaust gaspurification system of a sixteenth embodiment according to the presentinvention, wherein an central passage is composed of a hollow portion ofthe DPF;

[0159]FIG. 30 is a schematic composition drawing of the exhaust gaspurification system of the sixteenth embodiment according to the presentinvention, wherein a honeycomb structure provided separately is insertedin the hollow portion of the DPF;

[0160]FIG. 31 is a schematic composition drawing of the exhaust gaspurification system of the sixteenth embodiment according to the presentinvention, wherein a honeycomb structure is attached to an inlet member,and inserted in the hollow portion of the DPF during the mounting;

[0161]FIG. 32 is a composition diagram showing an example of acontinuous regeneration type DPF system where an oxidation catalyst ofthe prior art is disposed;

[0162]FIG. 33 is a composition diagram showing an example of acontinuous regeneration type DPF system provided with a DPF withoxidation catalyst of the prior art;

[0163]FIG. 34 is a composition diagram showing an example of acontinuous regeneration type DPF system provided with a DPF with PMoxidation catalyst of the prior art;

[0164]FIG. 35 is a schematic composition drawing of a wall flow typeDPF, (a) is a perspective view including a partial cross-section, (b) isa front view, and (c) is a back view;

[0165]FIG. 36 is a schematic lateral cross-section of the DPF of FIG.35;

[0166]FIG. 37 is a schematic isothermal diagram showing a state oftemperature distribution, during the regeneration of the DPF of theprior art; and

[0167]FIG. 38 is a lateral cross-section of an exhaust gas purificationsystem of the prior art to which the DPF of FIG. 35 is attached.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0168] Now, the exhaust gas purification system of the embodiments 1 to16 according to the present invention shall be described taking a casewhere the DPF is a wall flow type filer as example and referring todrawings.

[0169] First, the exhaust gas purification system of the embodiments 1to 8 concerning partial modifications of portions of the DPF whereparticulate matter (PM) tends to accumulate easily and where thetemperature tends to raise easily shall be described referring to FIG. 1to FIG. 17.

[0170] The wall flow type DPF used for the exhaust gas purificationsystem of the first and the second embodiments, as shown in FIG. 1 toFIG. 4, has a number of exhaust gas passages 11 a, 11 b whose peripheryis formed of porous wall surface 12 and, at the same time, is formed bystop sealing in zigzag 13 an inlet side 15 and an outlet side 16 of theexhaust gas passages (cells) 11 a, 11 b, and is used by incorporating inan exhaust gas purification systems 1A, 1B, 1C or others as shown inFIG. 32 to FIG. 34.

[0171] There, in these embodiments, it is composed by installing anexhaust gas passage prevention structure, against the exhaust gaspassages 11 a, 11 b in the central portion of the exhaust gas inflowportion of the DPF 10A, 10B.

[0172] It should be noted that, in FIG. 1 and FIG. 3, the exhaust gaspassage prevention structure is installed, against thirteen exhaust gaspassages 11 a, 11 b in the central portion.

[0173] First, the DPF 10A to be used for the exhaust gas purificationsystem of the first embodiment shown in FIG. 1 and FIG. 2 shall bedescribed.

[0174] In the composition, the exhaust gas passage prevention structureis composed by stop sealing 13A both the upstream side and thedownstream side of the cell 11 a, 11 b of the central portion C of theDPF 10A, so that the exhaust gas G cannot enter in the cells 11 a, 11 bof the central portion C thereof.

[0175] Thanks to the composition, as the exhaust gas G does not passthrough the cell 11 a, 11 b of the central portion C, PM in the exhaustgas G will not be collected and accumulated. Therefore, a local hightemperature due to combustion of the collected PM is prevented fromgenerating, avoiding the melding damage of the central portion C of theDPF 10A.

[0176] Also, the obstruction of the passage of the exhaust gas G makesthe exhaust gas G flowing toward the central portion C is directedaround the central portion C of the stop seal 13A, the whole exhaust gasG can be filtered by making it pass through a wall face 12 having thefiltering function of the DPF 10A.

[0177] Next, the DPF 10B to be used for the exhaust gas purificationsystem of the second embodiment shown in FIG. 3 and FIG. 4 shall bedescribed.

[0178] In the composition, an exhaust gas passage prevention structureis installed by forming the central portion C of the DPF 10B with asolid structure 17, and composing so that the exhaust gas passage beabsent in the central portion C.

[0179] Due to the composition, as there is no more passage for theexhaust gas G to pass, in the central portion C, PM also will notaccumulate no more.

[0180] According to the composition, effects similar to the DPF 10A usedfor the first embodiment can be expected, and furthermore, the formationof the solid structure 17 increases the heat capacity, allowing toabsorb and accumulate by the solid portion C heat generated throughcombustion of PM accumulated around the central portion C and also todiffuse by heat conduction, hence, a local high temperature around thecentral portion C is prevented.

[0181] Also, as the solid portion 17 has heat accumulating function andheat conducting function, and acts in a temporal and spatialhomogenization direction of the DPF temperature, by conductingaccumulated heat to the upstream side and the surrounding wall face,thus allowing to promote the combustion of PM in the exhaust gas G andcollected PM.

[0182] Next, the DPF for the exhaust gas purification system of third tofifth embodiments shown in FIG. 5 to FIG. 7, is composed by installing aheat absorbers 17C, 17D, 17E in the central portion C of the exhaust gasinflow portion and in the surrounding portion of the central portion C,of the DPFs 10C, 10D, 10E.

[0183] First, the DPF 10C to be used for the exhaust gas purificationsystem of the third embodiment shown in FIG. 5 shall be described.

[0184] In the composition, a heat absorber 17C formed by increasing thewall thickness of the porous wall face 12 is provided in a well curbshape, surrounding the central portion C of the exhaust gas inflow faceof the DPF 10C, namely portion where the temperature tends to raised,when the oxidation of accumulated particulate matter has started, duringthe regeneration mode operation or others.

[0185] Due to the composition, heat generated in the central portion Cis absorbed and accumulated by the heat absorber 17C, and conducted tothe outer peripheral portion, preventing the central portion C frombecoming partially hot, and the DPF 10C from melting damage.

[0186] Also, as heat accumulated during the DPF regeneration by the heataccumulating function of the heat absorber 17C is conducted to theupstream side and the surrounding wall face 12 by the heat conductingfunction of the heat absorber 17C, the temperature of the DPF 10C isdirected to a temporal and spatial homogenization direction, allowing topromote the combustion of particulate matter in the exhaust gas and thecombustion of collected particulate matter during the normal operation.Consequently, the amount of accumulation of particulate matterdecreased, the regeneration mode operation interval becomes longer,allowing to improved the durability and the life.

[0187] Next, the DPF 10D to be used for the exhaust gas purificationsystem of the fourth embodiment shown in FIG. 6 shall be described.

[0188] In the composition, a heat absorber 17D is provided cylindricallysurrounding the central portion C of the DPF 10D. The heat absorber 17Dis made of the same material as the porous wall face 12 and formedsimilarly, but heat accumulating function and heat conducting functionare improved compared to the porous wall face 12, by increasing the wallface thickness.

[0189] Due to the composition, heat generated in the central portion Cis absorbed and accumulated by the heat absorber 17D, preventing thecentral portion C from becoming partially hot, and thereby the meltingdamage of DPF can be avoided.

[0190] Next, the DPF 10E to be used for the exhaust gas purificationsystem of the fifth embodiment shown in FIG. 7 shall be described.

[0191] In the composition, a heat absorber 17E is provided crossing atthe central portion C of the DPF 10E. The heat absorber 17E is made ofthe same material as the porous wall face 12 and formed similarly, butheat accumulating function and heat conducting function thereof areimproved compared to the porous wall face 12, by increasing the wallthickness.

[0192] In accordance with the composition, the heat generated throughcombustion of PM accumulated around the central portion C cab beabsorbed and accumulated by the heat absorber 17E and diffused by theheat transmission, thereby preventing the surrounding of the centralportion C from heating locally.

[0193] Also, as the heat absorber 17E has heat accumulating function andheat conducting function, and acts in a temporal and spatialhomogenization direction of the DPF temperature, by conductingaccumulated heat to the upstream side and the surrounding porous wallface, thus allowing to promote the combustion of PM in the exhaust gas Gand collected PM.

[0194] Furthermore, in the composition, as the exhaust gas passage 11 a,11 b in the central portion C is reduced, PM accumulating in the centralportion C also reduces.

[0195] Though in the aforementioned description, the heat absorber 17C,17D, 17E is made of the same material as the porous wall face 12;however, it may also made of other material, for instance, stainlesssteel of metal material or ceramic material. In addition, concerning theshape, it may be molded integrally, or may be formed separately and thenintegrated by assembly.

[0196] The disposition of heat absorber 17C, 17D, 17E is not limited towell curb, lattice, concentric, ring or radical form, but it may also bedisposed in another form or in a combined form.

[0197] Then, the DPF of the exhaust gas purification system of the sixthto eighth embodiment shown in FIG. 8 to FIG. 16 is composed by providinga heat diffusion member 30F to 30L, against the downstream side of theDPF 10F, 10G.

[0198] The heat diffusing member to be used for the exhaust gaspurification system of the sixth embodiment is an aeration member 30Fsuch as aerating heat resistant metal mesh or porous flat plate usingalloy of aluminum, aluminum titanate or other metals, silicone carbide,alumina or others ceramics as material, abutted to the face of thedownstream side 16 of the DPF 10F. It may bonded by an adhesive, but itmay also be disposed so as to come into contact. Namely, it is sodisposed to improve the efficiency of heat conduction.

[0199] The heat diffusion member 30F may be a single plate, but aplurality of members may also be laminated. Especially, in the case ofmetal mesh, it is laminated preferably in order to secure an appropriateheat conduction characteristic and heat capacity.

[0200] If the composition is adopted, the difference of heat expansionbetween the DPF 10F and the heat diffusion member 30F can be absorbed bythe friction between planes each other.

[0201] There, according to the composition, as the heat can be diffusedin a direction to uniform the DPF heat distribution, especially radialheat distribution, and, particulate matter (PM) collection speed andcombustion speed, a temperature distribution as shown in FIG. 17 can beobtained. The comparison between the temperature distribution and theheat distribution of the DPF 10X of the prior art as shown in FIG. 37demonstrates a considerable improvement.

[0202] As shown in FIG. 10 and FIG. 11, the heat diffusing member to beused for the seventh embodiment is composed of a stop sealing plate 30Ghaving a stop sealing protrusion 13B disposed in zigzag, using alloy ofaluminum, aluminum titanate or other metals, silicone carbide, aluminaor others ceramics as material.

[0203] The stop sealing plate 30G is inserted and bonded from thedownstream side 16 of the DPF 10G, and the outlet portion 16 b of thedownstream side 16 of the DPF 10G is sealed in zigzag with the stopsealing protrusion 13G. In the composition, the bonding is realized byan adhesive 40, for absorbing the generation of gap due to thedifference of heat expansion between the DPF 10G and the stop sealingplate 30G.

[0204] There, according to the composition, the heat can be diffused ina direction to uniform the DPF heat distribution, especially radial heatdistribution, and, particulate matter (PM) collection speed andcombustion speed, by the stop sealing plate 30G.

[0205] Then, as shown in FIG. 12 to FIG. 17, the heat diffusing memberto be used for the eighth embodiment is composed by connection a wallflow type second DPF 30H, 30I, 30J formed by using aluminum alloy,aluminum titanate or other metals, silicone carbide, alumina or othersceramics as material, to the downstream side of a wall flow type firstDPF 10F, 10G composed of cordierite.

[0206] First, in a composition shown in FIG. 12 and FIG. 13, thedownstream side 16 of the first DPF 10G is not stop sealed, the secondDPF 30H is composed of a mesh of cell same as the first DPF 10G, theupstream side is not stop sealed and the position of the downstream sidewell curb-like stop seal 13′ is differentiated from the position of theupstream side stop seal 13 of the first DPF 10G. Namely, it is socomposed that the upstream side cell 11 a of the first DPF 10G and theupstream side cell 11 a′ of the second DPF 30H are connected.

[0207] On the other hand, in another composition shown in FIG. 14, afirst DPF 10F wherein an upstream side 15 and a downstream side 16 arestop sealed respectively, and a second DPF 30I wherein an upstream sideand a downstream side are also stop sealed are bonded with an adhesive40.

[0208] The second DPF 30I is composed of a mesh of cell same as thefirst DPF 10G, and formed so that the position of the upstream side wellcurb-like stop seal 13′ will be the same position as the position of thedownstream side stop seal 13 of the first DPF 10G. Namely, it is socomposed that the downstream side cell 11 b of the first DPF 10G and thedownstream side cell 11 a′ of the second DPF 30I are connected.

[0209] Then, another composition shown in FIG. 15 is substantially sameas the composition of FIG. 13, but an aeration member 30F which is aheat diffusion member is disposed between the first DPF 10F and thesecond DPF 30H. The radial heat conduction is promoted through theaeration member 30F.

[0210] Still another composition shown in FIG. 16 resembles to thecomposition in FIG. 14, and a stop seal plate 30K wherein stop sealprotrusions 13K, 13K′ are formed in zigzag on both face sides is bondedby adhesive 40 with the downstream side 16 of the first DPF 10F and theupstream side of the second DPF 30J. There, the radial heat conductionis promoted through the stop seal plate 30K.

[0211] And in these eighth embodiments, the boding by means of theadhesive 40 absorbs the difference of heat conductivity between thefirst DPFs 10F, 10G and the second DPFs 30H, 30I, 30J, or the aerationmember 30F, the stop seal plate 30K.

[0212] If the composition is adopted, the upstream side first DPF 10F,10G being composed of cordierite or other material showing a low heatconductivity, an excellent heat retention, and, a low heat capacity andan excellent heat elevation, become hot relatively rapidly, allowing topromote the combustion of PM. On the other hand, the downstream sidesecond DPFs 30H, 30I, 30J being composed of silicone carbide, alumina orother ceramic material presenting a high heat conductivity, an excellentdiffusivity, and, a large heat capacity and a mediocre heat elevation,allows to diffuse and accumulate heat, in order to avoid an abnormallyhigh temperature.

[0213] Also, the disposition of an aeration member 30F or stop sealpalte 30K which is a heat diffusion member between the first DPFs 10F,10G and the second DPFs 30H, 30J, can promote the radial heatconduction.

[0214] Next, the exhaust gas purification system of the ninth tofourteenth embodiments relating to the use of strong and weak catalystoxidation power shall be described referring to FIG. 18 to FIG. 25.

[0215] As shown in FIG. 18 to FIG. 20, the wall flow type filter 10L.10M, 10N has a number of exhaust gas passages (cells) 11 a, 11 b whoseperiphery is formed of porous wall surface 12 and is formed by stopsealing in zigzag 13 the inlet side 15 and the outlet side 16 of theexhaust gas passages 11 a, 11 b.

[0216] Next, a catalyst 30 a to 30 d made of oxidation catalyst, or,oxidation catalyst and PM oxidation catalyst is applied to the wallsurface of the exhaust gas passage 11 a, 11 b.

[0217] The oxidation catalyst is formed with a rear metal such asplatinum, and is a catalyst for oxidizing nitrogen monoxide (NO) intonitrogen monoxide (NO₂) with oxygen (O₂) in the exhaust gas, andoxidizes PM with the generated NO₂. Moreover, the PM oxidation catalystformed of cerium dioxide or the like, is a catalyst for directlyoxidizing the PM by activating the oxide in the exhaust gas.

[0218] The exhaust gas G enters the exhaust gas passage 11 a from theinlet side 15, passes through the porous wall face 12, enters theexhaust gas passage 11 b, and discharged from the outlet side 16 aspurified exhaust gas Gc. During the passage through the porous wall face12, PM in the exhaust gas, when it is hot, is purified through oxidationand combustion by the catalytic effect of the catalyst 30 a to 30 dapplied to the porous wall surface 12. When cold, it is collected by theporous wall surface 12. Moreover, the collected PM is eliminated throughoxidation and combustion by the catalytic effect of the catalyst 30 a to30 d during the regeneration control operation.

[0219] In the DPF 10L of the ninth embodiment shown in FIG. 18, theoxidation power of the catalysts 30 a, 30 b to be disposed in the DPF10L is composed so as to decrease in order and step-wise from theupstream to the downstream of the DPF 10L.

[0220] In short, as shown in FIG. 18, the DPF 10L is divided into anupstream side zone ZA1, an central portion zone ZA2 and a downstreamside zone ZA3, from upstream to downstream, a strong oxidation powercatalyst 30 a is disposed by coating in the upstream side zone ZA1, aweak oxidation power catalyst 30 b in the central portion zone ZA2respectively, while the downstream side zone (downstream end portion)ZA3 is not coated with catalyst, so as to decrease the oxidation powerin three stages.

[0221] The change of oxidation power of the catalyst 30 a, 30 b can berealized by changing the kind of catalyst, or even when the samecatalyst is used, by changing the support concentration during thecoating.

[0222] In the DPF 10M of the tenth embodiment shown in FIG. 19, theoxidation power of the catalysts 30 a, 30 b to be disposed in the DPF10M is composed so as to decrease in order and step-wise, from the outerperiphery towards the center of the DPF 10M.

[0223] In short, as shown in FIG. 19, the DPF 10M is divided into acylindrical outer peripheral side zone ZB1, an intermediate tubular zoneZB2 and a central side zone ZB3, from the outer peripheral side to thecentral side, a strong oxidation power catalyst 30 a is disposed bycoating in the cylindrical outer peripheral side zone ZB1, a weakoxidation power catalyst 30 b in the intermediate tubular zone ZB2respectively, while the central side zone (center portion) ZB3 is notcoated with catalyst, so as to decrease the oxidation power in threestages.

[0224] In the DPF 10N of the eleventh embodiment shown in FIG. 20, theoxidation power of the catalysts 30 a-30 d to be disposed in the DPF 10Nis composed so as to decrease in order and step-wise from the outerperipheral portion B of the upstream end to the central portion C of thedownstream end of the DPF 10N.

[0225] In short, as shown in FIG. 20, the DPF 10N is divided intoseveral zones ZC1 to ZC5 by division lines approximating a parabolawhich is convex at the upstream side, from the outer peripheral portionB of the upstream end to the central portion C of the downstream end,and strong oxidation power catalysts 30 a to 30 d are disposed bycoating consecutively, from the zone ZC1 near the outer peripheralportion of the upstream end. On the other hand, the zone ZC5 of thecentral portion of the downstream end is not coated with catalyst, so asto decrease the oxidation power in multiple stages (5 stages).

[0226] There, the DPF10L, 10M, 10N is used by incorporating in theexhaust gas purification system 1B, 1C or the like as shown in FIG. 33or FIG. 34, according to the kinds of catalyst 30 a to 30 d.

[0227] Next, the DPF regeneration control in the exhaust gaspurification system 1B, 1C or the like as shown in FIG. 33 or FIG. 34incorporating these DPF10L, 10M, 10N shall now be described.

[0228] The DPF regeneration control is performed according to aregeneration control flow as shown in FIG. 25. The regeneration controlflow is shown as a control flow to be executed in parallel with a maincontrol flow for controlling the engine, or others. Consequently, whenthe engine operation is started, the flow is called from the maincontrol flow to start, and the execution thereof is stopped, byinterruption, at the same time at the engine operation stop command suchas stopping the engine or others, it returns and goes back to the maincontrol flow. It should be noted that the execution suspension portionby the interruption is not illustrated in the regeneration control flowof FIG. 25.

[0229] There, when the control flow starts, the normal operation isperformed for a predetermined period of time in the step S11, beforegoing to the step S12, it is judged if the filter requires theregeneration control or not, and if unnecessary, it returns to thenormal operation of the step S11.

[0230] If it is judged that the filter regeneration control is requiredby the judgment in the step S12, it proceeds to the step S20 to performthe regeneration control operation.

[0231] In the regeneration control operation of the step S20, first, thedownstream exhaust gas temperature Tc of the DPF is monitored to judgeif it is lower than the set temperature Tc0 which would provoke DPFmelting damage or destruction, and catalyst deterioration.

[0232] The exhaust gas heat-up control for filter regeneration isperformed for a certain period of time and, in the judgment of the stepS21, in case where the downstream exhaust gas temperature Tc of the DPFis lower than the set temperature Tc0, the exhaust gas heat-up controlis set in the step S22 and it proceeds to the step S24, while in casewhere the exhaust gas temperature Tc exceeds the set temperature Tc0,the exhaust gas heat-up suspension is set in the step S23 and itproceeds to the step S24.

[0233] Then, the regeneration control operation is performed for acertain period of time in the step S24 according to the setting of thestep S22 or step S23, it is to terminate or not the regeneration controloperation and, if to terminate, it returns to the normal operation ofthe step S11 and if not to terminate, it returns to the step S21, tocontinue the regeneration control operation.

[0234] There, in the regeneration control operation, according to theDPF 10L, 10M, 10N of the aforementioned configuration, when oxidationand combustion of accumulated PM start, in portions of the DPF 10L, 10M,10N where the catalyst 30 a of strong oxidation power is disposed(portion ZA1 of the upstream side (inlet side), portion ZB1 of the outerperipheral side, portion of the upstream side (inlet side) and portionZC1 of the outer peripheral side), oxidation and combustion of PM start,by the strong oxidation effect.

[0235] Then, heat generated by the PM oxidation is supplied to portionswhere catalysts 30 b to 30 d of low oxidation power are disposed potion(portion ZA2 of the downstream side (after-flow side), intermediatetubular zone ZB2, central portion side portions ZC2 to ZC4 of thedownstream side) through heat conduction or others from the heated upexhaust gas, the upstream side or the outer peripheral side, even thecatalysts 30 b to 30 d of low oxidation power and weak oxidationactivate sufficiently, and the oxidation of PM is performed.

[0236] What is more, as the catalysts 30 b to 30 d of these portions,ZA2, ZB2, ZC2 to ZC4 are weak in oxidation power, a rapid PM oxidationis avoided, and melting damage, combustion loss, destruction of the DPF10L, 10M, 10N or catalyst deterioration are also prevented.

[0237] As exhaust gas purification system of the twelfth embodiment, anexhaust gas purification system 120A in the case of application to acontinuous regeneration type DPF system 1U of NO₂ regeneration type DPFsystem shown in FIG. 32 shall be described.

[0238] The exhaust gas purification system 120A shown in the FIG. 22 isa purifier using oxidation of particulate matter (PM) by NO₂, comprisingan upstream oxidation catalyst converter 3Aa and a downstream wall flowtype DPF 110A, wherein NO in the exhaust gas is oxidized by the upstreamoxidation catalyst converter 3Aa and PM collected by the downstream DPF110A is oxidized by the generated NO₂ into CO₂, to eliminate PM.

[0239] Here, the oxidation catalyst converter 3Aa is the one supportingan oxidation catalyst 32A (32As, 32Aw) in the exhaust gas passage of ahoneycomb structure made of cordierite or ceramics, formed in a tubularshape where the exhaust gas G enters the upstream end portion and exitsfrom the downstream end portion.

[0240] In this embodiment, as shown in FIG. 22 and FIG. 23, an outerperipheral side passage portion Z1 (hatched portion) of the oxidationcatalyst converter 3Aa supports a strong oxidation catalyst 32As made ofrear metal system oxidation catalyst or the like, while a central sidepassage portion Z2 (non hatched portion) supports a weak oxidationcatalyst 32Aw made of oxide system oxidation catalyst or the like.

[0241] In addition, the cross-section area of the outer peripheral sidepassage portion Z1 supporting the strong oxidation catalyst 32As is setto 0.5 to 1.0 times of the cross-section area of the central sidepassage portion Z2 supporting the weak oxidation catalyst 32Aw in orderto keep a good temperature distribution formation in the downstream sideDPF 110A, oxidation efficiency by the catalyst effect, thermal retentioncountermeasures or others in a good state.

[0242] And, the DPF 110A is also formed in a tubular shape where theexhaust gas G enters the upstream end portion and exits from thedownstream end portion. The DPF 110A is made of a wall flow type filter10X or others as shown in FIG. 35.

[0243] The wall flow type filter 10X, made of cordierite or ceramics,comprises a number of exhaust gas passages (cell) whose periphery isformed of porous wall surface 12 and an inlet side 15 b and an outletside 16 b of the exhaust gas passage 11 a, 11 b stop sealed in zigzag.

[0244] According to the exhaust gas purification system 120A of thecomposition, in the normal operation mode, as the outer peripheral sidepassage portion Z1 where the exhaust gas temperature tends to lowereasily by heat radiation or others supports the strong oxidationcatalyst, in the upstream side oxidation catalyst converter 3Aa, andtherefore, the oxidation effect is enhanced, the oxidation reaction ofPM or NO components or others contained in the exhaust gas is promoted,resulting in the elevation of the exhaust gas temperature.

[0245] At the same time, as the central side passage portion Z2 wherethe exhaust temperature tends to raise easily supports the weakoxidation catalyst, the oxidation effect is low, the oxidation reactionof PM or NO components or others contained in the exhaust gas isreduced, decreasing the elevation width of the exhaust gas temperaturedue to the oxidation reaction.

[0246] Consequently, the global temperature distribution of theoxidation catalyst converter 3Aa and the downstream side DPF 110A isequalized, in a way to enlarge the combustion area of PM in the exhaustgas, in the normal engine operation.

[0247] Also, in the regeneration mode operation, NO is produced inquantity and the exhaust temperature is raised, by changing the engineoperation state, but when the exhaust gas passes through the oxidationcatalyst converter 3Aa, the exhaust gas G in the outer peripheral sidepassage portion Z1 becomes hot through oxidation of NO and NO₂ isproduced in quantity and, therefore, collected PM burns actively and thetemperature raises rapidly in the outer peripheral side passage of theDPF 110A. Nonetheless, the outer peripheral side also cools down rapidlyby radiation. On the other hand, the exhaust gas G passing through thecentral side passage portion Z2 and flowing in the central side passageof the DPF 110A is relatively cold and contains little NO₂ and,therefore, the caught PM burns slowly, preventing an abnormally hightemperature.

[0248] Therefore, the temperature distribution in the DPF 110A is madeuniform wholly and the abnormal high temperature parts is not causedduring the regeneration mode operation, so the melting of the filter canbe avoided.

[0249] Next, as exhaust gas purification system of the thirteenthembodiment, an exhaust gas purification system 120B in the case ofapplication to a continuous regeneration type DPF system 1V of NO₂regeneration type DPF system shown in FIG. 33 shall be described.

[0250] The exhaust gas purification system 120B shown in the FIG. 24 isa purifier wherein oxidation catalyst 32A (32As, 32Aw) is applied to thewall surface of the wall flow type filter with catalyst 110B to achieveoxidation of NO in the exhaust gas and PM oxidation by NO₂ generated bythe oxidation, on the wall surface.

[0251] In this embodiment, a strong oxidation catalyst 32As made of rearmetal system oxidation catalyst or the like and a weak oxidationcatalyst 32Aw made of oxide system oxidation catalyst or the like areprepared, and as shown in FIG. 24, an outer peripheral side passageportion Z1 of the DPF 110B supports the strong oxidation catalyst 32As,while a central side passage portion Z2 supports the weak oxidationcatalyst 32Aw respectively.

[0252] In addition, the cross-section area of the outer peripheral sidepassage portion Z1 supporting the strong oxidation catalyst 32As is setto 0.5 to 1.0 times of the cross-section area of the central sidepassage portion Z2 supporting the weak oxidation catalyst 32Aw in orderto keep a good temperature distribution formation in the DPF 110B,oxidation efficiency by the catalyst effect, or others in a good state.

[0253] According to the exhaust gas purification system 120B of thecomposition, in the normal operation mode, as the outer peripheral sidepassage portion Z1 supports the strong oxidation catalyst 32As, theoxidation reaction of components such as NO or others contained in theexhaust gas in the DPF 110B is promoted, resulting in the elevation ofthe exhaust gas temperature, while in the central side passage portionZ2 supporting the weak oxidation catalyst 32Aw, the oxidation reactionand the elevation exhaust gas temperature are reduced, and consequently,the global temperature distribution of the DPF 110B is equalized.

[0254] Also, in the regeneration mode operation, the oxidation reactionof PM collected by DPF 110B is promoted in the outer peripheral sidepassage portion Z1 supporting the strong oxidation catalyst 32As andraises the filter temperature, while the oxidation reaction of collectedPM is slow in the central side passage portion Z2 supporting the weakoxidation catalyst 32Aw, and consequently, the global temperaturedistribution of the DPF 110B is equalized. As a result, the filtermelting damage due to an abnormally high temperature combustion that canhappen easily during the regeneration operation can be prevented.

[0255] Also, as exhaust gas purification system of the fourteenthembodiment, an exhaust gas purification system 120C in the case ofapplication to a continuous regeneration type DPF system 1W of NO₂regeneration type DPF system shown in FIG. 34 shall be described.

[0256] The exhaust gas purification system 120C shown in the FIG. 25 isa purifier wherein oxidation catalyst 32A (32As, 32Aw) and PM oxidationcatalyst 32B are applied to the wall surface of the wall flow typefilter with PM oxidation catalyst 110C to achieve oxidation of PM from atemperature lower than the wall surface.

[0257] The PM oxidation catalyst 32B is a catalyst for oxidizing PMdirectly with O₂ in the exhaust gas and is made of cerium dioxide orothers.

[0258] In the continuous regeneration type DPF system 1C, PM is oxidizedwith NO₂ using a reaction to oxidize NO of the oxidation catalyst 32Ainto NO₂, mainly, in the low temperature oxidation area (about 350° C.to 450° C.), PM is oxidized using a reaction to oxidize PM directly withO₂ in the exhaust gas O₂ Of the PM oxidation catalyst 32B in the middletemperature oxidation area (about 400° C. to 600° C.), while PM isoxidized with O₂ in the exhaust gas, in the high temperature oxidationarea (equal or superior to about 600° C.) higher than the temperature atwhich PM burns with O₂ in the exhaust gas.

[0259] In this embodiment, the oxidation catalyst 32A is composed sothat a strong oxidation catalyst 32As made of rear metal systemoxidation catalyst or the like and a weak oxidation catalyst 32Aw madeof oxide system oxidation catalyst or the like are prepared, and anouter peripheral side passage portion Z1 of the DPF 110C supports thestrong oxidation catalyst 32As and the PM oxidation catalyst 32B, whilea central side passage portion Z2 supports the weak oxidation catalyst32Aw and the PM oxidation catalyst 32B respectively.

[0260] In addition, the cross-section area of the outer peripheral sidepassage portion Z1 supporting the strong oxidation catalyst 32As is setto 0.5 to 1.0 times of the cross-section area of the central sidepassage portion Z2 supporting the weak oxidation catalyst 32Aw in orderto keep a good temperature distribution formation in the filter,oxidation efficiency by the catalyst effect or others in a good state.

[0261] According to the exhaust gas purification system 120C of thecomposition, the temperature distribution of the DPF 110C can beequalized, and as a result, the filter melting damage can be avoided bypreventing an abnormally high temperature from generating.

[0262] Also, the exhaust gas purification system of the fifteenth andsixteenth embodiments concerning the exhaust gas passage shall bedescribed referring to FIG. 26 to FIG. 31.

[0263] As shown in FIG. 26 to FIG. 31, DPFs 210A to 210F used for theexhaust gas purification system 20A to 20F is composed of a wall flowtype filter having a number of exhaust gas passages 11 a, 11 b whoseperiphery is formed of porous wall surface 12 and an inlet side 15 b andan outlet side 16 b of cells 11 a, 11 b constituting the exhaust gaspassage stop sealed in zigzag 13, to be used incorporated in an exhaustgas purification system 1U, 1V, 1W or others as shown in FIG. 32 to FIG.34. Consequently, the DPF 210A to 210F may be a normal DPF, or a DPFwith a catalyst such as oxidation catalyst, namely catalytic DPF.

[0264] In the exhaust gas purification system 20 of the fifteenthembodiment shown in FIG. 26, a DPF 210A made of a wall flow type filteris formed into a tubular body such as cylindrical form or the like andan outer peripheral passage 24A is disposed at the outer peripheralportion of the tubular body and a central passage 27A in the centralportion along the central axis respectively.

[0265] In addition, an outside chamber 25A communicating the outerperipheral passage 24A and the upstream side cell 11 a is installed, andmoreover, an inside chamber 26A communicating the downstream side cell11 b and the central passage 27A is provided. A gas inlet passage 23A isprovided outside the inside chamber 26A, to communicate the gas inlet21A and the outer peripheral passage 24A. A gas diffusion member 22A isdisposed at the gas inlet 21A, and composed to diffuse equally theinflow exhaust gas G in the outer peripheral passage 24A. It should benoted that an ash discharge port 34 is disposed in the downstreamportion of the outside chamber 25A in order to discharge ash accumulatedin the outside chamber 25A.

[0266] Then, the exhaust gas passage is composed so that the exhaust gasG passes through the gas inlet 21A, gas diffusion member 22 a, gas inletpassage 23A, outer peripheral passage 24A, outside chamber 25A, cell 11a, 11 b, inside chamber 26A, central passage 27A, and gas outlet passage28A in the order.

[0267] In short, it is so composed to make the exhaust gas G passthrough the outer peripheral passage 24A before flowing in the upstreamside cell 11 a, downstream side cell 11 b, and discharge the exhaust gasGc purified by the porous wall face 12 between the upstream side cell 11a and the downstream side cell 11 b out of the exhaust gas purificationsystem 20A via the central passage 27A.

[0268] According to the composition, the outer peripheral side of theDPF 210A can be heated or kept warm by the exhaust gas G, by flowing thetotal amount of exhaust gas G through the outer peripheral passage 24Aand then through the cells 11 a, 11 b of the DPF 210A, contributing to afurther homogenization of the temperature distribution of the DPF 210Aboth in the engine normal mode operation and the DPF regeneration modeoperation.

[0269] Next, in the composition of FIG. 26, the outer peripheral passage24A is formed as a passage between the outer periphery of the DPF 210Aand a case 31 of the exhaust gas purification system 20A, as a passageprovided outside the DPF 210A. In the case, the outer peripheral side ofthe DPF 210A is fixedly supported by an inner wall 26 a continuous to aninner chamber 26A through a DPF holding mat 32.

[0270] Thus, by adopting a composition of installing the outerperipheral passage 24A separately from the DPF 210A outside the DPF210A, the filter portion more vulnerable than the other portions can beprotected, and the expensive filter portion will not be damaged duringtransportation or installation of the exhaust gas purification system20A, even if the outside or the exhaust gas purification system 20A isdamaged.

[0271] Also, as shown in FIG. 27, the outer peripheral passage 24B canbe formed by removing stop seals before and after several layers ofcells 11 a′, 11 b′ in an inside area along the outer periphery of thetubular body of the DPF 210B. The composition of removing the stop seal13 can be formed relatively easily, because it can be molded into ashape from which the stop seal 13 is taken out during the molding of theDPF 210B. In the case, the outer peripheral side of the DPF 210B comesto be held through the DPF holding mat 32.

[0272] Next, an oxidation catalyst is disposed in the outer peripheralpassage 24B. The composition permits to heat the outer peripheral sideof the DPF 210B with oxidation reaction heat by the oxidation catalyst.Also, the space for disposing the oxidation catalyst can be economized.

[0273] Concerning the disposition of oxidation catalyst on the outerperipheral passage 24B, it may be supported by the porous wall face 12around the cell 11 a′, 11 b′ from which the stop seal 13 is taken out asshown in FIG. 27, or a porous support layer supporting the oxidationcatalyst may be laminated on the porous wall face 12.

[0274] Or, as shown in FIG. 28, the outer peripheral passage 24C isformed as a passage between the outer periphery of the DPF 210C and acase 31 of the exhaust gas purification system 20C, and in the passage24C, the oxidation catalyst is supported by a ceramic or metal honeycombstructure 41 formed separately from the DPF 210C, and the honeycombstructure 41 is disposed through insertion into the outer peripheralpassage 24C. In the case, the oxidation support step can be cut off fromthe molding step of the DPF 210C and, moreover, the oxidation catalystcan be disposed relatively easily in the outer peripheral passage 24C.

[0275] Also, as for the central passages 27A, 27B, 27C, as shown in FIG.26 and FIG. 27, an area along the central axis of a tubular body isformed hollow, and the hollow portion is taken as the central passage27A, 27B, or as shown in FIG. 28, the central passage 27C is formed byremoving stop seals before and after cells 11 a″, 11 b″ in an area alongthe central axis of the tubular body.

[0276] As the central passages 27A, 27B, 27C are formed in a portionincluding the downstream side of the central portion where an extremelyhigh temperate generates to create the DPF melting damage, in a DPF 10Xof the prior art of FIG. 35, the melting damage which was encountered inthe DPF 10X of the prior art can be avoided.

[0277] It should be noted that the combination of the composition of theouter peripheral passages 24A, 24B, 24C and the composition of thehollow passages 27A, 27B, 27C is made conveniently, compositions in FIG.26 to FIG. 28 are nothing but exemplary, and the present invention isnot limited to the combinations shown in these drawings.

[0278] Further, in the aforementioned exhaust gas purification systems20A, 20B, 20C, relief valves 33A, 33B, 33C are disposed so that theexhaust gas G bypasses the DPFs 210A, 210B, 210C, when the clogging ofthe DPFs 210A, 210B, 210C progresses and increases the pressure.

[0279] The relief valves 33A, 33B, 33C are disposed, as shown in FIG.26, on a passage wall 28 w between an outside chamber 25A communicatingthe outer peripheral passage 24A and the upstream side cell 11 a, andthe gas outlet passage 28A, or, as shown in FIG. 27, on a passage wall26 w between an inside chamber 26B communicating the downstream sidecell 11 b and the central passage 27B and the gas inlet passage 23B.Otherwise, as shown in FIG. 28, it may be disposed in a portion insidethe gas diffusion member 22C of the passage wall 26 w between an insidechamber 26C communicating the downstream side cell 11 b and the centralpassage 27C, and the gas inlet passage 23C.

[0280] The disposition of the relief valves 33A, 33B, 33C allows toavoid an excessive elevation of the engine exhaust pressure, bybypassing the filtration portion, when the DPFs 210A, 210B, 210C areclogged excessively by an abnormal accumulation of particulate matter.

[0281] In addition, if the relief valves 33A, 33B, 33C are disposed atthe position of FIG. 26 to FIG. 28, the piping for arranging the reliefvalves 33A, 33B, 33C becomes unnecessary, making the exhaust gaspurification system more compact.

[0282] Now, the exhaust gas purification system of the sixteenthembodiment shall be described.

[0283] In the exhaust gas purification system 20D of the sixteenthembodiment shown in FIG. 29, a wall flow type DPF 210D is formed into atubular body such as cylindrical form or the like and an outerperipheral passage 24D is disposed at the outer peripheral portion ofthe tubular body and a central passage 27D in the central portion alongthe central axis respectively.

[0284] In addition, an inside chamber 26D communicating the centralpassage 27D and the upstream side cell 11 a is provided, and an outsidechamber 25D communicating the downstream side cell 11 b and the outerperipheral passage 24D is installed. A gas outlet passage 23D isprovided outside the inside chamber 26D, to communicate the gas outlet28D and the outer peripheral passage 24D.

[0285] Then, in the sixteenth embodiment, as shown in FIG. 29, it is socomposed that the exhaust gas G passes through the gas inlet 21D,central passage 27D, inside chamber 26D, cells 11 a, 11 b, outsidechamber 25D, outer peripheral passage 24D, gas outlet passage 23D andgas outlet 28D in this order.

[0286] The composition of the sixteenth embodiment is different from thecomposition of the fifteenth composition in that the exhaust gas Gpasses through the central passage 27D before the cells 11 a, 11 b, andthen through the outer peripheral passage 24D, and, the portion whereoxidation catalyst is disposed in the central passage 27D.

[0287] Though, in the composition shown in FIG. 29 to FIG. 31, the outerperipheral passage 24D is disposed outside the DPF 210D as a passagebetween the outer periphery of the DPF 210D and the case 31; however,the outer peripheral passage may also be formed be removing stop sealsbefore and after the cell in the area along the outer periphery of thetubular body of the DPF, as is the case for the composition of FIG. 27.

[0288] As for the central passage 27D, as shown in FIG. 29, an areaalong the central axis of a tubular body may be formed hollow, and thehollow portion may be taken as central passage 27D, or though not shown,it can be formed by removing stop seals before and after cells in anarea along the central axis of the tubular body.

[0289] Next, as shown in FIG. 30, an oxidation catalyst is disposed inthe central passage 27E. In the FIG. 30, the oxidation catalyst issupported by a ceramic or metal honeycomb structure 42 formed separatelyfrom the DPF 210E, and the honeycomb 42 is disposed through insertioninto the central passage 27E of the DPF 210E. It should be noted that,in FIG. 31, the honeycomb 43 is disposed in connection with the membercomposing the gas inlet 21F, and composed to be inserted into thecentral passage 27F of the DPF 210F during the assembly of the exhaustgas purification system 20F.

[0290] In addition, though not illustrated, the central passage may beformed by removing stop seals before and after cells in an area alongthe central axis of the tubular body, and the oxidation catalyst can bearranged in the central passage, by providing on the porous wall surfacearound the cell along the central axis of the tubular body, or, bylaminating a porous support layer supporting oxidation catalyst on theporous wall surface.

[0291] Further, as shown in FIG. 31, disposition of a thermal insulationlayer 51 on the outer surface of the case 31 of the DPF 20F can enhancethe thermal retention effect and, at the same time, prevent thermaldamage of peripheral vehicle components disposed outside by the heat.Besides, the thermal insulation layer 51 having also a normal noisereduction effect can serve for prevention of noise.

[0292] In these compositions of the sixteenth embodiment, as the centralpassages 27D, 27E, 27F are upstream, the exhaust gas flowing in theouter peripheral passages 24D, 24E, 24F turns to be the purified exhaustgas Gc after having passed through the cell 11 a, 11 b, these exhaustgases G, Gc allow to retain heat and heat up the central portion and theouter peripheral portion of DPFs 210D, 210E, 210F.

[0293] In addition, if oxidation catalyst is disposed in the centralpassage 27E, the central portion of the DPF 210E can be heated with heatof the exhaust gas G and heat generated by oxidation reaction ofsubstances in the exhaust gas G.

[0294] And, in the aforementioned exhaust gas purification systems 20D,20E, 20F, a relief valve is disposed, so that the exhaust gas G bypassesthe DPFs 210D, 210E, 210F, when the clogging of the DPFs 210D, 210E,210F progress and increases the pressure.

[0295] The relief valves 33D, 33E, 33F are disposed, as shown in FIG.29, on a passage wall 21 w between an outside chamber 25D communicatingthe outer peripheral passage 24D and the downstream side cell 11 b, andthe gas inlet passage 21D, or, as shown in FIG. 30, on a passage wall 26w between an inside chamber 26E communicating the upstream side cell 11a and the central passage 27E, and the gas outlet passage 23E.Otherwise, as shown in FIG. 31, it may be disposed in a central portionof the passage wall 26 w between an inside chamber 26F communicating theupstream side cell 11 a and the central passage 27C, and the gas outletpassage 23F.

[0296] The disposition of the relief valves 33D, 33E, 33F allows toavoid an excessive elevation of the engine exhaust pressure, bybypassing the filtration portion, when the DPF s210D, 210E, 210F areclogged excessively by an abnormal accumulation of particulate matter.

What is claimed is:
 1. An exhaust gas purification system having adiesel particulate filter to purify particulate matter in the dieselengine exhaust gas, composed of an exhaust gas passage preventionstructure installed in a portion where the PM in the exhaust gas iscollected and accumulated easily and, at the same time, the temperatureraises easily by oxidation of the accumulated PM, in said dieselparticulate filter.
 2. The exhaust gas purification system of claim 1,wherein said diesel particulate filter is a wall flow type filter havinga number of exhaust gas passages whose periphery is formed of porouswall surface and inlet side and outlet side of the exhaust gas passagesealed in zigzag, and said exhaust gas passage prevention structure isinstalled against said exhaust gas passage of the central portion of theexhaust gas inflow cross-section of said filter and, at the same time,said exhaust gas passage prevention structure is stop sealed both at theupstream side and the downstream side of said exhaust gas passage. 3.The aforementioned exhaust gas purification system of claim 1, whereinsaid diesel particulate filter is a wall flow type filter having anumber of exhaust gas passages whose periphery is formed of porous wallsurface and inlet side and outlet side of the exhaust gas passage sealedin zigzag, and said exhaust gas passage prevention structure isinstalled against said exhaust gas passage of the central portion of theexhaust gas inflow cross-section of said filter and, at the same time,said exhaust gas passage prevention structure is made solid without theexhaust gas passage.
 4. An exhaust gas purification system having adiesel particulate filter to purify particulate matter in the dieselengine exhaust gas, composed of a heat absorber installed in a portionwhere the PM in the exhaust gas is collected and accumulated easily and,at the same time, the temperature raises easily by oxidation of theaccumulated PM, in said diesel particulate filter, or in a surroundingportion of the portion.
 5. The exhaust gas purification system of claim4, wherein said diesel particulate filter is a wall flow type filterhaving a number of exhaust gas passages whose periphery is formed ofporous wall surface and inlet side and outlet side of the exhaust gaspassage sealed in zigzag, and wherein said heat absorber is formed of athick porous wall surface in the central portion of the exhaust gasinflow cross-section of said filter or the thick porous wall surfacesurrounding the central portion.
 6. An exhaust gas purification systemhaving a diesel particulate filter to purify particulate matter in thediesel engine exhaust gas, composed of a heat diffusing member,presenting a higher heat conductivity than the diesel particulatefilter, disposed in contact with a portion where the PM in the exhaustgas is collected and accumulated easily and, at the same time, thetemperature raises easily by oxidation of the accumulated PM, in saiddiesel particulate filter.
 7. The exhaust gas purification system ofclaim 6, wherein said heat diffusing member is composed using metal,silicon nitride, and silicon carbide as material.
 8. The exhaust gaspurification system of claim 6 or 7, wherein said diesel particulatefilter is a wall flow type filter having a number of exhaust gaspassages whose periphery is formed of porous wall surface and upstreamside and downstream side of the exhaust gas passage sealed in zigzag,and wherein said heat diffusing member is composed of a stop sealingplate for sealing said downstream side in zigzag.
 9. The exhaust gaspurification system of claim 6 or 7, wherein said heat diffusing memberis composed of a member having a filter function.
 10. The exhaust gaspurification system of claim 9, wherein both of said diesel particulatefilter and said heat diffusing member are composed of a wall flow typefilter having a number of exhaust gas passages whose periphery is formedof porous wall surface, and wherein the upstream side and the downstreamside of said exhaust gas passage are sealed in zigzag.
 11. An exhaustgas purification system having a diesel particulate filter to purifyparticulate matter in the exhaust gas from a diesel engine by using acatalyst, wherein the oxidation power of said catalyst to be disposed inthe diesel particulate filter is composed so as to decrease in order,step-wise or continuously, from the upstream to the downstream of thediesel particulate filter.
 12. An exhaust gas purification system havinga diesel particulate filter to purify particulate matter in the exhaustgas from a diesel engine by using a catalyst, wherein the oxidationpower of said catalyst to be disposed in the diesel particulate filteris composed so as to decrease in order, step-wise or continuously, fromthe outer periphery towards the center side of the diesel particulatefilter.
 13. An exhaust gas purification system having a dieselparticulate filter to purify particulate matter in the exhaust gas froma diesel engine by using a catalyst, wherein the oxidation power of saidcatalyst to be disposed in the diesel particulate filter is composed soas to decrease in order, step-wise or continuously, from the outerperiphery of the upstream end towards the central portion of thedownstream end of the diesel particulate filter.
 14. The exhaust gaspurification system of one of claims 11 to 13, wherein the oxidationpower of said catalyst is changed in three or more stages, or,continuously and, at the same time, the portion of the lowest oxidationpower of said catalyst is formed by not disposing said catalyst.
 15. Theexhaust gas purification system of one of claims 11 to 14, wherein saiddiesel particulate filter is a wall flow type filter having a number ofexhaust gas passages whose periphery is formed of porous wall surfaceand, an inlet side and an outlet side of an exhaust gas passage sealedin zigzag.
 16. An exhaust gas purification system comprising anoxidation catalyst converter in the upstream of the exhaust gas passageand a diesel particulate filter to purify particulate matter in theexhaust gas from a diesel engine in the downstream, wherein saidoxidation catalyst converter and said diesel particulate filter areformed into a cylinder-shape in which the exhaust gas enters in theupstream end portion and exits from the downstream end portion and, atthe same time, it is so composed that the outer peripheral passageportion of said oxidation catalyst converter supports a strong oxidationcatalyst, and the central passage portion a weak oxidation catalystrespectively.
 17. An exhaust gas purification system having a dieselparticulate filter to purify particulate matter in the exhaust gas froma diesel engine in the exhaust gas passage and disposing an oxidationcatalyst in the exhaust gas passage of said diesel particulate filter,wherein the diesel particulate filter is formed into a cylinder-shape inwhich the exhaust gas enters in the upstream end portion and exits fromthe downstream end portion and, at the same time, it is so composed thatthe outer peripheral passage portion of the diesel particulate filtersupports a strong oxidation catalyst, and the central passage portion ofthe diesel particulate filter supports a weak oxidation catalystrespectively.
 18. The exhaust gas purification system of claim 16 or 17,wherein said strong oxidation catalyst is made of a rear metal systemoxidation catalyst, while said weak oxidation catalyst is made of anoxide system oxidation catalyst.
 19. The exhaust gas purification systemof claim 18, the transversal cross-section area of the outer peripheralpassage portion supporting said strong oxidation catalyst is made into0.5 to 1.0 time of the transversal cross-section area of the centralpassage portion supporting said weak oxidation catalyst.
 20. The exhaustgas purification system of one of claims 16 to 19, wherein said dieselparticulate filter is a wall flow type filter having a number of theexhaust gas passages whose periphery is formed of porous wall surfaceand an inlet side and an outlet side of the exhaust gas passage sealedin zigzag.
 21. An exhaust gas purification system, wherein the dieselparticulate filter to purify particulate matter in the exhaust gas froman engine is composed of a wall flow type filter having an inlet sideand an outlet side of a plurality of cells whose periphery is formed ofporous wall surface sealed in zigzag, and wherein an outer peripheralpassage is disposed in the outer peripheral portion of said dieselparticulate filter and a central passage in the central portionrespectively, and it is so composed that exhaust gas passes through thegas inlet passage, said outer peripheral passage, said cells, saidcentral passage and the gas exit passage in this order.
 22. The exhaustgas purification system of claim 21, wherein the oxidation catalyst isdisposed in said outer peripheral passage.
 23. The exhaust gaspurification system of claim 22, wherein the oxidation catalyst issupported by a ceramic or metal honeycomb structure formed separatelyfrom the diesel particulate filter and the honeycomb structure isdisposed in said outer peripheral passage, for disposing an oxidationcatalyst in the outer peripheral passage.
 24. The exhaust gaspurification system of one claims 21 to 23, wherein a relief valve isdisposed at least one of a passage wall between an outside chambercommunicating said outer peripheral passage with said cell and said gasexit passage, or a passage wall between an inside chamber communicatingsaid cell and said central passage, and said gas inlet passage, so thatthe exhaust gas bypasses said filter, when the clogging of said filterprogresses and increases the pressure.
 25. An exhaust gas purificationsystem, wherein the diesel particulate filter to purify particulatematter in the exhaust gas from an engine is composed of a wall flow typefilter having an inlet side and an outlet side of a plurality of cellswhose periphery is formed of porous wall surface sealed in zigzag, andwherein an outer peripheral passage is disposed in the outer peripheralportion of said diesel particulate filter and a central passage in thecentral portion respectively, and it is so composed that exhaust gaspasses through the gas inlet passage, said central passage, said cells,said outer peripheral passage and the gas exit passage in this order.26. The exhaust gas purification system of claim 25, wherein anoxidation catalyst is disposed in said central passage.
 27. The exhaustgas purification system of claim 26, wherein, the oxidation catalyst issupported by a ceramic or metal honeycomb structure formed separatelyfrom the diesel particulate filter and said honeycomb structure isdisposed in said central passage, for disposing an oxidation catalyst insaid central passage.
 28. The exhaust gas purification system of one ofclaims 21 to 23, wherein a relief valve is disposed at least one of apassage wall between an outside chamber communicating said cell and saidouter peripheral passage and said gas inlet passage, or a passage wallbetween an inside chamber communicating said central passage and saidcell, and said gas exit passage, so that the exhaust gas bypasses saidfilter, when the clogging of said filter progresses and increases thepressure.
 29. The exhaust gas purification system of one of claim 21 to28, wherein a thermal insulation layer is disposed on the outer surfaceof said exhaust gas purification system.